Near-end-member magnesiochloritoid in prograde-zoned pyrope, Dora-Maira massif, western Alps

Abstract

In the ultrahigh-pressure metamorphic terrane of the Dora-Maira massif, western Alps, magnesiochloritoid (up to 97 mol% end-member) occurs along with ellenbergerite, talc, chlorite or kyanite as prograde inclusions in a few pyrope megablasts. Whereas most pyrope megablasts in this terrane are relatively homogeneous (Prp90-Prp98), the magnesiochloritoid-bearing megablasts commonly preserve a clear prograde FeMg zonation pattern (from Prp70 to Prp98). The nearly homogeneous chloritoid inclusions also record this zonation pattern, their X Mg increasing from 0.70 to 0.96 from garnet core to garnet rim. These zonations are shown to be prograde and to record increasing P and T conditions up to about 700°C, 30 kbar. The occurrence of magnesiochloritoid in some garnets simply reflects either a slightly more Fe-rich system (general case), or a more Al-rich and originally corundum-bearing system (one unzoned near-end-member pyrope megablast). FeMg partitioning between chloritoid and zoned host-garnet changes from normal to reverse (i.e., X Mg Cld < X Mg Grt) from core to rim, or from rim to core in other megablasts of yet similar composition. In case of reverse partitioning, a diffusion zone of a few tens of μm wide exists in garnet around the inclusions, toward which X Mg Grt decreases and reaches a normal partitioning value at the contact. These features are interpreted in terms of disequilibrium and tentatively ascribed to slow diffusion in chloritoid with respect to garnet growth rate, or to two competing garnet-forming reactions, with the metastable one, which produces a more Mg-rich garnet, being kinetically favoured. The prograde inclusions of magnesiostaurolite show a consistent FeMg reversal with garnet, i.e. X Mg St < X Mg Grt, which is interpreted as an equilibrium feature at high pressure (and high X Mg). Magnesiochloritoid locally breaks down into symplectites of chlorite and an extremely low-Si magnesiostaurolite (down to 7.0 Si per formula unit), which is probably metastable with respect to magnesiostaurolite + corundum.